Hybrid electric vehicles (HEV) can selectively utilize different power sources to optimize fuel economy. The power sources are typically an internal combustion engine and a high-voltage electric motor/generator unit (MGU), or multiple MGUs, and may be arranged in series or in parallel with respect to each other to provide various vehicle operating modes. In a hybrid vehicle, the engine can be automatically turned off whenever the HEV is at a standstill or while coasting to minimize idle fuel consumption, and can be quickly restarted via the MGU or an auxiliary starter motor when a driver of the HEV requests forward or reverse travel, e.g., by depressing an accelerator pedal.
In an automatic transmission, a hydrodynamic torque converter replaces a mechanical friction clutch for the purpose of transferring input torque from either or both power sources to an input member of the transmission. A torque converter assembly thus provides a fluid coupling between power sources and the transmission, while also multiplying torque at low vehicle speeds and providing a mechanical connection for efficiency via an internal friction clutch. In some vehicle designs, an electric torque converter assembly may be used in lieu of a hydrodynamic torque converter to replace certain hydraulic components of the latter with electro-mechanical devices, while still providing a similar level of input coupling and torque multiplication.